The announcement of Rantissi's election was made over loudspeaker during a gathering of tens of thousands of Hamas supporters at a soccer stadium in Gaza City, a day after Hamas founder Sheik Ahmed Yassin was assassinated by Israel.

Rantissi said that Hamas would press for more attacks against Israel. "We will be unified in the trenches of resistance," he said. "We will not surrender, we will never surrender to Israeli terror."

Followers of Sheikh Yassin and his Muslim militant group the swift transition shows that Hamas is not a cult of personality which will wither away now that its charismatic leader is dead. The wheelchair-bound cleric carved out a persona that blurred the lines between the bloody politics of suicide bombings and Islamic piety.

Israel has killed more than 20 Hamas operatives since last September. And Defense Minister Shaul Mofaz declared Tuesday that the Israeli military would try to eradicate the entire leadership of Hamas, whose suicide bombing have killed about 400 Israelis over the past three and a half years.

"Our leader is a system. It's a leadership by group, and anyone can represent the desires of the group," says Mahmoud Zahar, a senior official in Hamas and one of its founders, as he received a stream of visitors at a massive mourning in a tent here Tuesday. Nearby, official posters of Yassin offer sympathies from the Palestinian Authority of Yasser Arafat, whose peace deals with Israel in the 1990s put him at odds with Hamas's rejection of all negotiations.

"There will be no vacuum in our organization," quips Mr. Zahar, a short, bearded man who maintains a medical practice and had been considered a strong candidate for ascendancy in Hamas. Israel, he suggests, overestimated the tactical damage they could do to Hamas by assassinating its chief leader. "Sheikh Yassin was absent for many years while he was in jail, and Hamas was able to continue in its activities," he says.

Israel has made it clear that the gloves are off. All Hamas leaders are targets for assassination, Prime Minister Ariel Sharon's government has deemed. "Everyone is in our sights," Internal Security Minister Tsahi Hanegbi told reporters Tuesday. "There is no immunity for anyone."

But the death of Yassin, many analysts believe, will only serve to increase the lure of Hamas and serve as a de facto recruitment call for young Palestinians.

"I would say that Sharon, by sponsoring this targeted assassination of Yassin, has strengthened Hamas. It has certainly not damaged its ability to operate," says Henry Siegman, a Middle East expert on the Council on Foreign Relations in New York.

"I don't believe that Hamas depended on the charisma and character of Yassin for its ability to attract people to kill themselves," says Mr. Siegman. "There is no bureaucracy, there is no structure there. It has more to do with the occupation and the rage against it, and those conditions haven't changed."

Zahar, who survived an assassination attempt by Israel last year in which his son was killed, acknowledges he is a marked man. The only man who is higher on the list is Abdel Aziz Rantissi, one of the co-founders of the organization and one who was closely associated with Yassin.

While Zahar is more in tune with political developments and has shown more willingness to work with the Palestinian Authority, Mr. Rantissi's reputation is of a hard-liner who is more closely tied to Hamas's military wing.

Rantissi was chosen Tuesday by a meeting of the 12-member leadership council, or shura, which Hamas leaders say does not coordinate with the "military wing" of the organization, the Qassem Brigades, which plan attacks - primarily in the form of suicide bombings against Israeli civilians.

Ghazi Hamad, a former member of Hamas who remains closely linked to Islamic political groups, says that Yassin's word was not the be-all, end-all of activity in Hamas. Sometimes, Mr. Hamad says, others in Hamas would disagree with the sheikh. Though the organization is now missing a unifying personality, he says, it will hardly be paralyzed from acting against Israel exactly as it did before.

"I don't expect there will an essential change inside Hamas," says Hamad, at the offices of Al Risale, an Islamist newspaper where he serves as editor.

But to insiders, Yassin had the ability to coax people toward more moderate positions: He was responsible for encouraging Hamas militants to obey the temporary hudna, or cease-fire, last summer. In a recent interview, Yassin said he was willing to stop attacks on Israel from the Gaza Strip if Mr. Sharon made a full withdrawal from the territory. Going forward, in a Hamas without Yassin it could be much harder to get all supporters of the organization on board with a certain policy.

"They believe that if they destroy the leaders of Hamas, they can destroy the resistance," says Hamad. "But this will not break the hand of Hamas."